Wheel bearing apparatus has been improved in order to reduce its size and weight to improve fuel consumption of the vehicle while reducing its manufacturing cost. The wheel bearing apparatus has a unit structure including a wheel hub and a rolling bearing. A wheel bearing apparatus of a so-called third generation type integrally comprises an inner member and an outer member along with a flange. The inner member comprises a wheel hub and an inner ring press-fit onto the wheel hub. One of the double row inner raceway surfaces is directly formed on the wheel hub and the other of the double row inner raceway surfaces is formed on the inner ring.
FIG. 14 illustrates a known wheel bearing apparatus of the third generation type for a driving wheel intended to have improved durability while assuring proper bearing pre-load during straight-advancing travel of the vehicle as well as bearing rigidity during curving or turning travel. The wheel bearing apparatus comprises an inner member 51, an outer member 60 and double row balls 56, 56 rollably contained between the inner and outer members 51, 60. The inner member 51 includes the wheel hub 52 and a separate inner ring 53 press-fit onto the wheel hub 52. The wheel hub 52 is integrally formed on its outer side end with a wheel mounting flange 54. Hub bolts 55, for securing a wheel, are mounted on the wheel mounting flange 54 equidistantly along its periphery.
The wheel hub 52 is formed with an inner raceway surface 52a on its outer circumference. A cylindrical portion 52b axially extends from the inner raceway surface 52a. The inner ring is formed with an inner raceway surface 53a on its outer circumference. The inner ring is press-fit onto the cylindrical portion 52b and axially secured thereon by a caulked portion 52c. The caulked portion 52c is formed by plastically deforming the end of the cylindrical portion 52b radially outward.
The outer member 60 is integrally formed with a body mounting flange 60b on its outer circumference. The outer member 60 is also formed with double row outer raceway surface 60a, 60a on its inner circumference. Double row balls 56, 56 are rollably contained, via cages 57, 57, between the inner and outer raceway surfaces 52a, 53a and 60a, 60a under an applied predetermined contact angle α (FIG. 15). Seals 58, 59 are arranged on both ends of the outer member 60 to prevent leakage of lubricating grease sealed within the bearing. The seals 58, 59 also prevent the entry of rain water or dust from the outside into the bearing.
A constant velocity universal joint 61 has an outer joint member 65 integrally formed with a cup shaped mouth portion 62, a shoulder portion 63, and a shaft portion 64. The shoulder portion 63 forms the bottom of the mouth portion 62. The shaft portion 64 axially extends from the shoulder portion 63. The shaft portion 64 of the outer joint member 65 is inserted into the inner member 51 via a torque transmittable mechanism. The outer joint member 65 is separably secured to the wheel hub 52 by a securing nut 67 fastened onto a male thread 66 formed on the shaft portion 64 after the shaft portion 64 is inserted into the wheel hub 52 until the shoulder portion 63 abuts against the caulked portion 52c of the wheel hub 52.
As shown in FIG. 15(a) the cross-section of the inner raceway surface 53a of the inner ring 53 is formed by two circular arcs of different radii of curvatures. That is, the inner raceway surface 53a of the inner ring 53 is formed by a circular arc with a radius of curvature “r”, and a circular arc with a radius of curvature “r1”. The radius of curvature “r” extends from a position near a contact point P that contacts the ball 56 at an initial contact angle α to a raceway bottom. The circular arc with a radius of curvature “r1” extends from a position near the contact point P to an outer circumference of a larger diameter of the inner ring 53.
As shown in FIG. 15(a) by cross-hatchings, the radius of curvature r is set larger than r1 and has a range 1.05 d<2 r≦1.10 d (wherein “d” is a diameter of the ball 56). The radius of curvature r1 is set within a range of 1.01 d≦2 r1≦1.05 d. Accordingly, as shown in FIG. 15 (b), the inner raceway surface 53a contacts with the ball 56 at the contact point P via the initial contact angle α and a predetermined pre-loading level during steady travel (straight advancing travel) of the vehicle. Also, the inner raceway surface 53a contacts with the ball 56 at the contact point P1 via the contact angle α1 and a predetermined pre-load level during curving travel of the vehicle where a moment load is applied to the wheel bearing apparatus.
As shown in FIG. 15(a), the outer raceway surface 60a of the outer member 60 is also formed by two circular arcs with different radii of curvature r2, r3. The circular arc with the radius of curvature r2 extends from a position near a contact point Q contacting the ball 56 at an initial contact angle α to a raceway bottom. The circular arc with the radius of curvature r3 extends from a position near the contact point Q to an inner circumference 60c of the outer member 60.
As shown in FIG. 15 (a) by cross-hatchings, the radius of curvature r2 is set larger than r3 and has a range 1.07 d<2 r2≦1.12 d (wherein “d” is a diameter of the ball 56). The radius of curvature r3 is set within a range 1.03 d≦2 r3≦1.07 d (wherein “d” is a diameter of the ball 56). Accordingly, as shown in FIG. 15 (b), the outer raceway surface 60a contacts with the ball 56 at the contact point Q at the initial contact angle α and with a predetermined pre-loading level during steady travel (straight advancing travel) of the vehicle. Also, the outer raceway surface 60a contacts the ball 56 at the contact point Q1 at the contact angle α1 and with a predetermined pre-loading level during curving travel of the vehicle where a moment load is applied to the wheel bearing apparatus.
Accordingly, it is possible to assure the initially applied pre-load and thus prevent an increase of the rolling resistance while suppressing an excessive pre-load during steady travel as well as to increase the pre-load level in accordance with the increase of moment load and thus improve the durability of the bearing rigidity without adding any part during curving travel of the vehicle. See, Japanese Laid-open Patent Publication No. 009895/2006.
Although the prior art wheel bearing apparatus is structured so that the inner and outer raceway surfaces are contacted by the ball, at a predetermined pre-load level in accordance with the contact angle varying according to the travelling conditions of the vehicle, generally a moment load is applied to the bearing and the initial contact angle α gradually increases and shifts to α1 during curving travel of the vehicle. However, for example, in the inner raceway surface 53a of the inner ring 53 formed by the two radii of curvature r, r1, since an inflection point (transition) of the two different radii of curvature r, r1 is not clearly set, it is believed that an excessive pre-load will be caused during steady travel of the vehicle or the pre-load would be insufficient during curving travel of the vehicle. Thus, it is required to set a proper inflection point.
In addition, in such a wheel bearing apparatus, since the inner raceway surface 53a is formed by two circular arc sections of two different radii of curvatures r, r1, a grinding work must be carried out by using a formed grinding wheel having a configuration of the inner raceway surface 53a. Accordingly, several kinds of formed grinding wheels are required to manufacture corresponding wheel bearings of different size specifications. Thus, the manufacturing cost, including an administrative expense of the wheel bearing, would be increased.
It is, therefore, an object of the present disclosure to provide a wheel bearing apparatus for a vehicle that can improve its durability by assuring a proper pre-load during straight advancing travel of the vehicle and also by assuring a proper bearing rigidity during curving travel of the vehicle as well as reduce the manufacturing cost by flexibly corresponding to the bearing specifications.